The use of spherical expandable bearings in spherical holes of bearing plates is widespread. In a particular use, the bearing plate serves as a fracture fixation plate adapted for being secured at one end to a bone fragment on one side of a bone fracture and a post is inserted through the bearing into another bone fragment on an opposite side of the fracture to stabilize it. The post also serves as a means to expand the bearing and lock it in place in an adjusted angular position in the hole in the plate. Reference is made to co-pending U.S. patent application Ser. No. 10/754,462 now U.S. Pat. No. 7,195,633 which discloses such an arrangement.
It has been found that with a standard spherical hole in the bearing plate, the wall of the hole is stiff and unyielding. Consequently, as the bearing expands, initially the bearing takes up a clearance with the wall of the hole and there is no contact with the wall. Hence, there is no reactive force to fix the bearing in the hole. As soon as contact with the hole is made by the expanding bearing, the reactive forces increase substantially instantaneously with no significant yielding of the wall of the hole. Because the hole is so stiff and unyielding, the bearing cannot be expanded any further.
In the event that microgrooves are provided on the surface of the bearing, these may become deformed and if the outer diameter of the bearing diminishes by even a small amount as the edges of the microgrooves are deformed, significant loss of wall reactive force may occur.